a) Field of the Invention
The invention is directed to a device for transporting magazines holding wafer-shaped objects formed of a magazine container which can be deposited with its a releasable bottom and a depositing surface of its wall on a holding plate of an enclosure for a clean room, wherein the holding plate has a lowerable plate part supporting the bottom with the wafer magazine. The magazine container is a SMIF box whose use is favored in the fabrication of semiconductor chips.
b) Description of the Related Art
Systems known as SMIF (Standard Mechanical Interface) systems contain a so-called SMIF box as a magazine container with an enclosed volume in which wafer magazines can be stored and transported. The SMIF box has a releasable bottom and is set down upon a holding plate of an enclosure for a clean room in such a way that adhering dust particles are trapped between the bottom of the SMIF box and a plate part which can be lowered into the clean room. The box itself surrounds the opening made in the enclosure.
A transport device of the type mentioned above is known, for instance, from U.S. Pat. No. 5,169,272.
Because of increasingly exacting demands for quality, a number of process steps in semiconductor chip fabrication require that work be carded out within the enclosure in a gas atmosphere adapted to the production process. The quality of this gas atmosphere has a decisive influence on the production results. In order to avoid disturbing the gas atmosphere when opening the box, it is necessary to produce within the box identical atmospheric conditions or a state in which the gas atmosphere is not influenced.
With the object of ensuring such adapted conditions, U.S. Pat. No. 5,169,272 describes a process and a device which provide a stepwise lowering of the releasable bottom accompanied by generation of pressure differentials for the purpose of cleaning the SMIF box. The generated pressure differential prevents gas from entering the clean room during the lowering process or while cleaning the SMIF box by means of guiding gas in the region of the bottom from one side of the SMIF box to the other.
A disadvantage in this solution consists in that the flow of gas through the interior of the box is incomplete, so that there is no assurance that a required concentration gradient will be achieved within an acceptable time period.
On the other hand, high-temperature processes, for instance, require removal of oxygen. Since the nitrogen conventionally used for purging is heavier than the oxygen or oxygen-nitrogen mixture, an adequate change in the concentration of nitrogen to oxygen may not be possible. If the box and magazine are purged with gas in an airlock, the consumption of gas increases by a multiple and the purging time is substantially prolonged.
U.S. Pat. No. 4,724,874 provides a duct which connects the interior region of the SMIF box directly with an exterior region. A vacuum or over-pressure can be generated in the SMIF box and gas can be transported into the interior via the duct. This technical solution is not suitable for carrying out an effective exchange of gas within the SMIF box. In spite of the high cost, a relatively long period of time is needed to achieve a permissible residual concentration of harmful gas in a process gas.
Further, EP 273 226 A2 discloses a closable dust-tight transport container with standardized outer dimensions whose semiconductor wafer holder is arranged within the inner hood so as to be movable without friction. Openings in an outer base plate and inner base plate and a filter element at the inner base plate serve to ensure particle-free air flow under fluctuating pressure ratios. Means serving only to compensate for pressure are not suitable for generating within the transport container a gas atmosphere adapted to the production process.